How to deal with a compulsive liar?

My boyfriend told me the craziest things and is always telling me he loves me, that he only wants me, what there's only me in his life, that he wouldn't even text other girls...

I got a friend to text him because I had the feeling he was lying. And I was right. He told her he was single but getting with this other girl (not me but his ex) and fucking another one (not me either but just a fling). And I know he isn't lying about this. But he made up so many things and told her too... And I don't know what to do.

So long story short, he's cheating and telling the crazist lies. Should I tell her his new friend was just a trap and break up or wait, plan something bad and make him suffer a bit more?